The present invention relates to a method of improving the adhesion of wear resistant surface coatings on whisker-reinforced ceramics resulting in improved performance of the coated product.
Cutting tool inserts made from SiC-whisker-reinforced alumina are an established product on the cutting tool market and are mainly used in cutting heat resistant materials and, to some extent, for use in cutting cast iron. However, when used for cutting steel and other iron-rich alloys SiC-whisker-reinforced alumina exhibits poor tool life time. It is believed that these poor tool lives are due to a strong chemical reaction which occurs between the iron in the steel and the silicon carbide whiskers in the cutting insert at the high temperatures produced at the cutting edge during machining. From, e.g., U.S. Pat. No. 4,801,510, it is known that this kind of attack is reduced by an application of a wear resistant surface coating. This improves the whisker-reinforced ceramic cutting tool insert's performance in the cutting of steel and other iron-rich alloys. However, it is difficult to provide sufficient adherence of the surface coating and, for that reason, the area of application of coated SiC-whisker-reinforced alumina cutting inserts has been restricted.
Hot isostatic pressing (HIP-treatment) using various encapsulating methods to facilitate the isostatic pressing operation is well-known. For example, U.S. Pat. No. 4,820,663 discloses a method of manufacturing a shaped, whisker-reinforced ceramic body of near theoretical density comprising forming a compact, encapsulating the compact by applying a coating by vapor deposition, heating and isostatically pressing the encapsulated compact in an atmosphere that reacts with the coating and/or compact substrate to increase the density of the resulting body. To make a cutting tool from that body, the coating is ground away and the body is shaped into a cutting tool insert having a cutting edge forward at the junction of a rake face and a flank face.